Conveyor belt-tensioning means

ABSTRACT

An endless conveyor has belt-tensioning means comprising two devices adapted and arranged to sense and signal variations in the tensions in the working and return runs of the belt respectively due to change of load conditions, a drum around which the return run of the belt passes and which is displacable so as to vary the tensions in the belt, speed-variable driving gear adapted to effect said displacement of the drum, and means adapted to effect actuation of said driving gear in response to signals emitted by said devices when said tensions are not at a predetermined ratio, and to effect variation in the speed of said gear, and hence of the displacement of the drum, in accordance with the signal characteristics which vary in accordance with the rate at which tension change in the belt takes place due to variation in load conditions.

United States Patent Comley [54] CONVEYOR BELT-TENSIONING MEANS [72]Inventor: Peter Donald Heywood Comley, Glasgow,

England {73] Assignee: Movor & Coulson Limited, Great Britain [22]Filed: Jan. 29, 1971 21 App1.No.: 110,827

[58] Field of Search ..l98/208, 16; 254/172;

[ Feb. 1, 1972 3,537,573 11/1970 Tangye et al. ..l98/208 PrimaryExaminerEvon C. Blunk Assistant ExaminerAlfred N. Goodman AnorneyMason,Fenwick & Lawrence [57] ABSTRACT An endless conveyor has belt-tensioningmeans comprising two devices adapted and arranged to sense and signalvariations in the tensions in the working and return runs of the beltrespectively due to change of load conditions, a drum around which thereturn run of the belt passes and which is displaca ble so as to varythe tensions in the belt, speed-variable driving gear adapted to effectsaid displacement of the drum, and means adapted to effect actuation ofsaid driving gear in response to signals emitted by said devices whensaid tensions are not at a predetermined ratio, and to effect variationin the speed of said gear, and hence of the displacement of the drum,

in accordance with the signal characteristics which vary in ac- [56]References Cited cordance with the rate at which tension change in thebelt UNITED STATES PATENTS takes place due to variation in loadconditions.

3,275,126 9/1966 Hartzell 198/208 7 Claims, 3 Drawing Figures l2 2 7 f/0 r5 r0 9 7 7 Q a I Inventor PETER Dmmw EYWooo Comm mm Q W A Home y gPATENIEB FEB I872 inventor Pew: bum Hevwoou CM.

By WQMJWQ m CONVEYOR BELT-TENSIONING MEANS This invention relates to anendless conveyor and to tensioning means for the endless member thereof,hereinafter referred to as a belt.

Most known belt conveyors include means for tensioning the conveyor beltand such means usually comprises a carriage carrying a drum over whichthe belt passes. The carriage is normally referred to as a loop carriageand is coupled to a cable to which tension may be applied by means ofweights acting under the influence of gravity, or by a powered winch.

Such known tensioning means takes up slack belt in the system, andautomatically adjusts according to the laden conditions of the conveyor,i.e., with increasing load, belt stretch occurs which is taken up by thetensioning means hauling in the loop carriage, and, conversely, withdecreasing load, belt contraction occurs, and the belt is released by anappropriate amount.

However, all such known tensioning means have to cater for the maximumbelt stretch that occurs under full load conditions, and are thereforetensioned for this peak load condition. This means that, whenever theconveyor is only lightly loaded, or even empty, the belt isovertensioned for the conditions pertaining.

A further aspect of winch-operated tensioning means is the rate ofoperation of the winches. It is important that slack in the belt istaken up by the tensioning means at a rate commensurate with the rate atwhich the slack is being formed.

Slack belt is normally formed at a maximum rate when starting upconveyor in the fully laden condition, and this rate may well be to 30times the rate of stretch for small-load changes.

The winches, however, are at present designed to operate at a speedcommensurate with the maximum rate of stretch, and consequently operateat too high a speed for all operating conditions other than the fullyladen start. In order to mitigate the possibility of hunting occurringwhen this speed is higher than demanded by operating conditions it issometimes necessary to build in a damping system, thus adding expenseand complication to the tensioning equipment.

An object of the present invention is to provide conveyortensioningmeans that automatically tensions the belt to a tension commensuratewith the loading conditions, and preferably operates at speedscommensurate with the rate at which belt stretch, or contraction,occurs.

According to the present invention we provide an endless conveyor havingbelt-tensioning means comprising two devices adapted and arranged tosense and signal variations in the tensions in the working and returnruns of the belt respectively due to change of load conditions, a drumaround which the return run of the belt passes and which is displaceableso as to vary the tensions in the belt, speed-variable driving gearadapted to effect said displacement of the drum, and means adapted toeffect actuation of said driving gear in response to signals emitted bysaid devices when said tensions are not at a predetermined ratio, and toefiect variation in the speed of said gear, and hence of thedisplacement of the drum, in accordance with the signal characteristicswhich vary in accordance with the rate at which tension change in thebelt takes place due to variation in load conditions.

Preferably, said driving gear includes a swashplate pump of which thetilt of the swashplate is controlled by said signal responsive means.

Preferably also, the arrangement is such that when the ratio of thetensions is that predetermined, the signals are of equal magnitude, andthe degree of tilt of the swashplate equal magnitude, and the degree oftilt of the swashplate increases with the degree of imbalance betweenthe signals.

Preferably also, the movement of said drum is effected by a winch drivenby a hydraulic motor fed by said pump.

Each of said devices may be a load cell of hydroelastics or electricalform.

Preferably, said devices are actuated by pivotal frames carrying rollersaround which the working and return runs of the belt pass respectively.

Said signal responsive means may be an electrohydraulic servovalve, inwhich the electrocontrol is a servomotor. The theory of the invention isbased upon the belt-drive formula T/r=e" where T= Tight side tension ofbelt I Slack side tension of belt 2 Constant p.= Coefficient of frictionbetween belt and drive drums 0= Angle of wrap of belt around drivedrums.

Now for any given drive, e, p. and 0 remain constant for all loadingconditions, therefore 7 and I will be directly proportional to eachother, irrespective of the load carried by the conveyor.

If these two tensions are measured in the form of signals, and comparedwith one another, they should be in balance when the tensions are at thecorrect ratio for the drive. An increase in load on the conveyor causesan increase in T tension with consequent increase in its signal output,the two signals then being out of balance. This out-of balance can beconverted to a voltage of, say, positive form, and by suitableamplifrcation used to pilot operate the tension winch to take in belt inorder to restore the correct ratio of T/r tensions.

Conversely, a decrease in load on the conveyor causes a decrease inT-tension with consequent reduction in its signal output, the twosignals then being out of balance. This out of balance is againconverted to a voltage, but of negative form, and by suitableamplification is used to pilot operate the tension winch to pay out beltso as to restore the correct ratio of TI! tensions.

An embodiment of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows a side view of a conveyor head and tensioning meansaccording to the present invention;

FIG. 2 shows the tensioning sensing and signalling devices of FIG. 1 toa larger scale, and

FIG. 3 shows the winch and driving gear.

The belt conveyor itself is of well-known form and its upper run 1passes over a jib drum 2 before passing to a driving head 3. Behind thedriving head 3 of the conveyor, a loop 4 is formed in the conveyor beltwhich passes over a drum 5 mounted upon a loop takeup carriage 6. Thecarriage 6 is capable of limited travel by means of a cable 7 which isattached to a winch 8, to be described in greater detail below. The beltthen passes from the loop over a drum 9 and continues to the tail end ofthe conveyor. The working run 1 of the belt is subject to Ttensions, andthe return run 10 of the belt is subject to t tensions.

The jib drum 2 is mounted in a frame 11, and the frame 11 is pivoted ona hinge pin 12 from a jib box 13. The lower end of the pivoted drumframe 11 rests against a load-measuring or sensing and signalling device14, which may be a load cell either of hydroelastic or electrical formwhich is mounted on the jib box 13.

With this arrangement,'a predetermined proportion of the T tension towhich the jib drum 2 is subjected, is supported and measured by the loadcell 14.

In a similar manner the drum 9 is mounted on a frame 15, and the frame15 is pivoted on a hinge pin 16 from a structure 17. The lower end ofthe pivoted drum frame 15 rests against another load-measuring orsensing and signalling device 18, which may be a load cell either ofhydroelastic or electrical form which is mounted on the structure 17.

With this arrangement a predetermined proportion of the t tension towhich the drum 9 is subjected, is supported and measured by the loadcell 18.

The load cells 14 and 18 are calibrated so that their outputs are ofequal magnitude when the tensions T and t are in the proportion T/t=e#for the particular drive concerned.

The outputs of the load cells l4, 18 are monitored and compared one withthe other. If the tension in belt run 1 increases, the load on the cell14 increases, in proportion, and this then puts the signal out ofbalance with the signal from cell 18. This out of balance may beconsidered to be of positive form, and is suitably amplified to signalthe loop winch 8 to take in the belt so as to restore equilibriumbetween T and t for the new conditions.

Conversely, if tension in the belt run 1 decreases, the load on the cell14 decreases in proportion, and this again puts the signal out ofbalance with the signal from the cell 18. This out of balance may beconsidered to be of negative form, and is suitably amplified to signalthe loop winch 8 to pay out belt so as to restore equilibrium between Tand t for the new conditions.

The amplified signal of positive or negative form is employed to operatean electrohydraulic servo valve 19, in which the electrocontrol may be aservomotor.

The control valve 19 is directly coupled to the swashplate tilt shaft(not shown) of a swashplate pump 20, whose swashplate 21 is capable oftilt to each side of the vertical so as to provide duo directionalvariable output.

The pump 21, which is driven by an electric motor 22, supplies oil to aslow speed, high-torque hydraulic motor 23, which is coupled direct tothe drum 24 of the winch 8. The pump and motor operate on a closedhydraulic circuit.

The loop carriage 6 is coupled to the winch unit 8 via a rope and pulleysystem, one method being as indicated in FIG. 3. Two rope pulleys 25 areattached to a pivot frame 26, which, in turn, is attached to the loopcarriage 6 via a pivot pin assembly 27. This arrangement ensured correcttracking of the loop carriage over the full extent of its travel.

The rope 7 is attached to the winch drum 24, the end portions of therope being wound on to end portions of the drum 24, and the middleportion of the rope passing around the pulleys 25 and a pulley 28, whichis attached centrally to the winch frame 8. With the arrangement asshown, slack belt is taken up by winding the rope on to the winch drum24, and hauling the loop carriage towards the winch 8. Conversely, beltis paid out by winding rope off the winch drum 24 and allowing thecarriage to move away from the winch 8.

When the winch 8 is not operating, the winch drum 24 is held stationaryby a brake 29, indicated diagrammatically, which may be spring or weightapplied ON, and hydraulically powered OFF.

Whenever the winch is operated, initial hydraulic pressure in the systemis employed to release the brake, thus the brake fails safe in the eventof hydraulic failure, and is not normally released until the pumpswashplate 21 has been actuated by an out of balance signal from theTand t load cells l4, l8, and consequently the loop is always under fullcontrol of the winch.

It can be seen from the above description that the embodiment providesan automatic loop control that tensions the belt run 10 in proportion tothe loading on belt run 1, so that the predetermined tension ratio T/:can be maintained substantially constant.

Now the strength of the out of balance signal between the load cells 14and 18 directly determines the angle through which the swashplate 21 istilted in pump 20, thus determining the output from the pump 20 and thespeed of winch operation.

if there is a very rapid change in balance between the two loadmeasuring devices, such as occurs when starting up a laden conveyor,then there is a large signal output with consequent rapid operation ofthe loop winch to restore equilibrium. As equilibrium is being restoredthe out of balance force diminishes progressively, bringing theswashplate 21 back towards the vertical (or no output) position, slowingdown the loop winch 8 until the winch is slowed to a stop as thetensions come back into balance and the swashplate 21 comes vertical.

If there is only a small change in out of balance between theload-measuring devices 14, 18, such as occurs when the load on the beltis increasing or decreasing, then there is only a proportionately smallmovement of the swashplate 21 with low pump output and slow winchoperation.

Thus, it can be seen that also 111 the embodiment described the rate oftakeup or paying-out is commensurate with the rate at which belt slackor contraction is formed.

What is claimed is:

1. An endless conveyor having belt tensioning means comprising, twodevices adapted and arranged to sense and signal variations in thetensions in the working and return runs of the belt respectively due tochange of load conditions, a drum around which the return run of thebelt passes and which is displaceable so as to vary the tensions in thebelt, speed-variable driving gear adapted to effect said displacement ofthe drum, and means adapted to effect actuation of said driving gear inresponse to signals emitted by said devices when said tensions are notat a predetermined ratio, and to effect variation in the speed of saidgear, and hence of the displacement of the drum, in accordance with thesignal characteristics which vary in accordance with the rate at whichtension change in the belt takes place due to variation in loadconditions.

2. An endless conveyor having belt-tensioning means as claimed in claim1 wherein, said driving gear includes a swashplate pump of which thetilt of the swashplate is controlled by said signal responsive means.

3. An endless conveyor having belt-tensioning means as claimed in claim2 wherein, the arrangement is such that when the ratio of the tensionsis that predetermined, the signals are of equal magnitude, and thedegree of tilt of the swashplate increases with the degree of imbalancebetween the signals.

4. An endless conveyor having belt-tensioning means as claimed in claim3 wherein, the movement of said drum is effected by a winch driven by ahydraulic motor fed by said pump.

5. An endless conveyor having belt-tensioning means as claimed in claim4 wherein, each of said devices may be a load cell of hydroelastics orelectrical form.

6. An endless conveyor having belt-tensioning means as claimed in claim5 wherein, said devices are actuated by pivotal frames carrying rollersaround which the working and return runs of the belt pass respectively.

7. An endless conveyor having belt-tensioning means as claimed in claim6 wherein, the amplified signal from the load cells of positive ornegative form is employed to operate an electrohydraulic servovalve inwhich the electrocontrol may be a servomotor.

UNITED STATES PATENT oFFIcE CETIFICATE F QEQ'HQN Patent No. 3,638 78lDated Eebruarv l 1972 Inventor(s) Peter Donald Hevwood Comlev It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

First page, left hand column, in the line identified by designator code[72] change the address of the inventor to read -Glasgow, GreatBritain-, and in the line identified by designator code [73] the name ofthe assignee should read --Mavor & Coulson Limited Signed and sealedthis 19th day of September 1972a (SEAL) Attest:

EDWARD M.FLETCHER JRo ROBERT GOT'ISGHALK Attesting Officer Commissionerof Patents ORM PO-IOSO (10-69) USCOMM-DC 60376-P69 x: us GOVERNMENTPRINTING OFFICE: I969 0-366-334

1. An endless conveyor having belt tensioning means comprising, twodevices adapted and arranged to sense and signal variations in thetensions in the working and return runs of the belt respectively due tochange of load conditions, a drum around which the return run of thebelt passes and which is displaceable so as to vary the tensions in thebelt, speed-variable driving gear adapted to effect said displacement ofthe drum, and means adapted to effect actuation of said driving gear inresponse to signals emitted by said devices when said tensions are notat a predetermined ratio, and to effect variation in the speed of saidgear, and hence of the displacement of the drum, in accordance with thesignal characteristics which vary in accordance with the rate at whichtension change in the belt takes place due to variation in loadconditions.
 2. An endless conveyor having belt-tensioning means asclaimed in claim 1 wherein, said driving gear includes a swashplate pumpof which the tilt of the swashplate is controlled by said signalresponsive means.
 3. An endless conveyor having belt-tensioning means asclaimed in claim 2 wherein, the arrangement is such that when the ratioof the tensions is that predetermined, the signals are of equalmagnitude, and the degree of tilt of the swashplate increases with thedegree of imbalance between the signals.
 4. An endless conveyor havingbelt-tensioning means as claimed in claim 3 wherein, the movement ofsaid drum is effected by a winch driven by a hydraulic motor fed by saidpump.
 5. An endless conveyor having belt-tensioning means as claimed inclaim 4 wherein, each of said devices may be a load cell ofhydroelastics or electrical form.
 6. An endless conveyor havingbelt-tensioning means as claimed in claim 5 wherein, said devices areactuated by pivotal frames carrying rollers around which the working andreturn runs of the belt pass respectively.
 7. An endless conveyor havingbelt-tensioning means as claimed in claim 6 wherein, the amplifiedsignal from the load cells of positive or negative form is employed tooperate an electrohydraulic servovaLve in which the electrocontrol maybe a servomotor.